The present invention relates to a lighting apparatus for an electric discharge lamp which operates to energize the electric discharge lamp and, more particularly, to a circuit arrangement of a lighting apparatus for an electric discharge lamp which is suitable to drive the lamp with a high efficiency.
In a lighting apparatus for an electric discharge lamp having a separately excited inverter apparatus which is equipped with an oscillating section and converts a DC electric power to an AC electric power by the output of this oscillating section, the power supply to this oscillating section is generally obtained by a method whereby level of the AC power is reduced to the voltage level that is needed for the oscillating section by a stepdown transformer and then it is rectified by a full wave rectifier. However, in this method, the stepdown transformer and full wave rectifier are necessary to obtain the power for the oscillating section, so that there is a drawback such that the circuit scale is large and expensive. There is another method whereby such a power is directly derived through a resistor after the AC power is rectified. However, this method has drawbacks such that the resistance value becomes large when the voltage is high and the electric power which is consumed by the resistor increases.
In addition, in a separately excited inverter apparatus, the lighting electric power is continuously supplied irrespective of the lighting state of the electric discharge lamp since the oscillating section continuously operates during the time interval when the power supply is turned on. The continuation of operation of such an inverter apparatus causes the light electric power generated to be consumed in vain and also causes a high voltage to be developed while the discharge lamp is lit off.
On the other hand, as a lighting apparatus for an electric discharge lamp having a self-excited inverter apparatus equipped with a base feedback winding, there have been proposed an apparatus in which no oscillation occurs even when the power supply is turned on in the case where the electric discharge lamp is removed, and an apparatus which stops the oscillation in the case where the preheating electrode is disconnected. These apparatuses are disclosed in Japanese Utility Model Publication Laid-open No. 15978/73 and Japanese Patent Publication Laid-open No. 3313/79, etc. However, with these arrangements, in a case where the oscillation has once started, the oscillation does not stop even if the discharge lamp is removed after the light-up or even if the discharge lamp is lit off due to the disconnection of the preheating electrode. Also, although it is possible to detect the defective assembly and disconnection of the preheating electrode of one discharge lamp, the defective assembly and disconnection of the other preheating electrode cannot be detected, so that the service life of the lamp will have been shortened and the unstable operation will have been continued, and the like. Therefore, the conventional lighting apparatuses still have various problems left that have to be solved.